Cell and clonal density, lacunar and clonal diameters, and mean number of cells per single clone were studied in human femoral condylar cartilage of normal and osteoarthrotic joints. The values were related to the age of the subjects, the sampling site within the joints, and the depth from the articular surface. Cell density in every zone of both normal and diseased tissue decreased with increasing distance from the surface and increasing age. Cell density was significantly lower in age-matched osteoarthritic articular cartilage with an intact surface than in that of normal joints. The age-related decrease in the cell density accompanied an increase in the density of empty lacunae. The density of Alcian blue-stained cells, which actively synthesize proteoglycans as demonstrated by histoautoradiography, diminished with advancing age in all zones of articular cartilage. The percentage of these cells consistently decreased in the superficial and increased in the deep layers of the tissue in both normal and osteoarthritic joints. The clonal density was higher in the nonweight-bearing than in weight-bearing areas, and it increased with age, whereas the mean clone diameter and clonal cell number decreased.